Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and output voltage, long cycle life and low self-discharge ratio are commercially available and widely used. Recently, with considerably increased demand for portable electric and/or electronic devices, demand for secondary batteries also rapidly increased and, especially, lithium secondary batteries comprise the majority of the secondary battery markets.
In addition, with the recent trend toward high performance and miniaturization of portable electric and/or electronic devices, various types of batteries with reduced size as well as high performance are required.
For laptop computer, since battery size greatly influences computer thickness, attempts to develop novel battery structures having decreased thickness as well as achieving high capacity and high performance have been made. Specifically, increased concern over environmental issues has brought about a great deal of research associated with electric vehicles (EV) and hybrid electric vehicles (HEV) as substitutes for vehicles using fossil fuels, such as gasoline vehicles and diesel vehicles, which are a major cause of air pollution.
In conventional lithium secondary batteries, a carbon material is usually used as an anode active material and use of lithium metal, a sulfur compound, or the like, is also considered. Meanwhile, lithium cobalt oxide (LiCoO2) is most commonly used as the cathode active material and, in addition, other lithium transition metal oxides including, e.g., lithium manganese oxides such as LiMnO2 having a layered structure, LiMn2O4 having a spinel structure, etc., lithium nickel oxides such as LiNiO2, are also used.
In order to increase capacity per unit mass, it is important to develop an improved high capacity active material. However, recently developed active materials are already close to theoretical capacity, thus limiting increase in capacity.
As an alternative method, setting an operating voltage of existing active material to a high voltage may be considered to extend the ranges of charge voltage and discharge voltage, thus increasing capacity of the active material. However, conventional active materials tend to deteriorate performance of a battery at high voltage and entail a problem of shortened lifespan due to side reaction at high voltage. For instance, if charge voltage of a lithium secondary battery is continuously maintained in a high voltage state, electrolyte pyrolysis, reaction of a lithium-containing anode with electrolyte, cathode oxidation, pyrolysis of a cathode active material, or the like may occur. Consequently, serious problems may be encountered in terms of battery safety.
Therefore, there is a need for techniques to increase battery capacity by operating the battery under high voltage conditions without causing the foregoing problems.